Low voltage spark drill

ABSTRACT

A spark drill for subterranean drilling is disclosed which may be attached to a standard or continuous drill string. The drill is characterized by multiple electrodes having a layer of semiconductor material between them which provides an electrical shunt path and further includes a trigger circuit for initiating the spark.

I United States Patent 1 [111 3 708 022 Woodruff [4 1 Jan. 2, 1973 54LOW VOLTAGE SPARK DRILL 3,679,007 5/1970" 2,953,353 9/1960 [75]Inventor. gleorge N. Woodrufl, Merritt lsland, 3,500,942 3/19702,963,620 l2/l960 Knudson et al ..l23/l69 R [73] Assignee: TRW lnc.,RedondoBeach, Calif.

Primary Examiner-James A. Leppink [22] led: June 1971 Attorney-Daniel T.Anderson, William B. Leach and 211 Appl. No.: 150,371 Harry Jacobs [57]ABSTRACT [52] US. Cl ..l75/16 51 Int. Cl. ..E2lb 7/00 A spark subemmea"[58] Field of Search 175/2 l1 19 123/169, which may be attached to astandard or continuous 315/35 drill string. The drill is characterizedby multiple electrodes having a layer of semiconductor material betweenthem which provides an electrical shunt path [56] References cued andfurther includes a trigger circuit for initiating the UNITED STATESPATENTS Spark- 3,l58,207 11/1964 Rowley ..l75/l6 14 Claims, 5 DrawingFigures 1 2| l l x i i8 PATENTEDJAM 2 I975 SHEET 1 [)F 2 r I 1. A

Rectification Circuit George N. Woodruff BY WW2? ATTORNEY PATENTEDJAN21m 3.708.022

SHEET 2 0F 2 George N. Woodruff Fl 5 INVENTOR.

BY Law/52 ATTORNEY LOW VOLTAGE SPARK muu.

BACKGROUND OF THE INVENTION The present invention relates generally todrilling equipment and more particularly to spark discharge drillsdesigned to create pressure waves in a drilling fluid which fracture thesubterranean structure.

Spark discharge drills and other drill types are generally discussed inthe book publication entitled Novel Drilling Techniques by William C.Maurerv, Pergamon Press, 1968. The present state of spark drilltechnology is also shown in the U.S. Pat. to Smith, Jr., No. 3,500,942,issuedMar. I7, 1970.

In general, drilling tools excavate rock by one of four basicmechanisms, namely, mechanically induced stresses, fusion andvaporization, and chemical reactions. The spark drill is one of severaldrills that fall into the class of drills operating to producemechanically induced stresses. The spark drill requires a working fluidthat fills the region between the electrodes and the work surface.Sparks discharged between the electrodes are believed to create pressurewaves which impact the work surface thereby transmitting energy into thesubterranean structure causing it to fracture.

The fractured structure is then flushed away by a fluid that isgenerally pumped downward within the drill string to the work region andrecirculated to the surfacealong the exterior of the drill string. Inspark drill technology, it is typical to have the flushing fluid serveas the working fluid in which the pressure or shock waves are generated.

Spark drills are particularly advantageous because they efficientlytransfer input energy to the subterranean structure. A primarydisadvantage of such drills has heretofore been the necessity for highvoltages which typically are of a 50 kilovolt magnitude. Voltages ofthis magnitude are difficult to handle and control in oil explorationfields. Furthermore, excessive losses would be incurred in transmittingsuch energy levels of direct current from surface equipment to a sparkdrill which may be several thousand feet beneath the surface.

Spark plug technology includes the use of a semiconductor materialbetween the spark plug electrodes to prevent spark plug fouling andfinds particular application in the aircraft fields where reliability isan important factor. A device of this type is shown in the U.S. Patentto Knudson et al., No. 2,963,620, issued Dec. 6, 1959.

Applicant has discovered that the use of a semiconductor shunt pathbetween the electrodes of a spark drill not only permits the generationofa spark, but provides certain nonfouling characteristic when operatedin a liquid and further enables the operation of the spark drill at muchlower voltage levels, namely in the order of magnitudeof 5 kilovolts andlower/This has important implications in the field of subterraneandrilling technology. For instance, these voltage levels are presentlyfound in many drilling applications. Also, present technology permits ofefficiently transmitting such low voltage alternating current voltagelevels to the drill. This being the case, the associated sparkinitiating circuitry having means for rectification, energy storage, anddischarge may be incorporated in the drill portion or another portion ofthe drill string near the drill. The initial breakdown gap voltagesheretofore required to operate a spark drill in fluids would requiresuch a large capacitor that space limitations would prohibit suchcircuitry at the drill.

While spark drills are known to provide higher drilling rates thanconventional drills, the high voltages required have made themimpracticable for competitive use in view of insulation requirements andenergy leakage. The, higher drilling rates associated with the practicalimplementation of the spark drill as disclosed herein providesexceptional economic advantages, for example, high drilling rates cutrental time which may range from one to thirty thousand dollars per day.

It is accordingly an object of the present invention to provide a sparkdrill for subterranean drilling which is not subject to the foregoingproblems and shortcomings of the prior art.

Another object of the present invention is to provide a spark drillwhich requires relatively low voltages for operation. I I

Still another object of the present invention is to provide a sparkdrill assembly which includes spark initiating circuitry.

SUMMARY OF THE INVENTION The presentinvention, contemplates a sparkdrill for subterranean drilling which comprises at least a pair ofelectrodes between which is provided a shunt path composed ofsemiconductor material. The drill further includes means for conductinga drilling fluid into the region between the electrodes and the worksurface. While various configurations may be constructed, the basicscheme permits the use of relatively low operating voltages.Furthermore, circuitry may be located in close proximity to the drillfor converting low voltage alternating current to a relatively highvoltage direct current spark.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a view in elevation and partialcross section of a spark drill accordance with the principles of thepresent invention;

FIG. 2 is a plan view of the lower end of the spark drill shown in FIG.1;

FIG. 3 schematically illustrates a block diagram of an electroniccircuit suitable for use as the trigger circuit for the spark drill ofFIG. 1; and

FIGS. 4 and 5 are a diagram of an alternative embodiment of the sparkdrill shown in FIGS. 1 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingsin which like characters refer to like parts, and in particular to FIGS.1 and 2 there is shown a spark drill 10 particularly suitable forattachment to the lower end of a drill string as used in oilexploration. The spark drill 10 includes a cylindrically shaped housing11 which serves as one of at least 2 electrodes. The housing orelectrode 11 is threaded at its upper end for threaded engagement withthe lower end of the drill string.

. The spark drill further includes a second or central electrode 12which is characterized by a plurality of radially extending legs 13. Theouter and central electrodes are insulated one from another by adielectric 14, except for thelower portions thereof. The radiallydirected paths from the central electrode 12 to the outer electrode 11is characterized by a layer of semiconductor material 15. It may benoted that applicant prefers to have the semiconductor layer recessedslightly from the lower surfaces of the electrodes. Each of the radiallyextending legs 13 taken in combination with the outer electrode 11provides a spark gap. In operation, there is a random pattern of sparksoccurring between the electrodes.

The spark drill further includes a drilling fluid supply tube 16 alsohaving mating threads 17 for engagement with a supply line originatingat the earths surface. The fluid supply tube 16 terminates in aplurality of drilling fluid distribution tubes 18.

In operation, the spark drill is introduced into the earth and a viscousdrilling fluid, sometimes called mud, is pumped downward through thesupply tube 16 and distribution tubes 18 to fill the region between thesubterranean structure and the electrodes. The fluid is allowed to flowupward along the exterior of the spark drill and drill string carryingwith it broken particles of the subterranean structure. It is thencommon practice to collect the drilling fluid and recycle it.

In spark drills heretofore known, a sufficient voltage is appliedbetween the electrodes to cause a discharge to occur between theelectrodes. This discharge, when created at a high repetition rate, isbelieved to form shock waves that travel to and impact upon thesubterranean structure immediately below the drill. Drills of this typehave been experimentally shown to have superior drilling ratecapabilities. However, such drills generally require high voltages, inthe order of 50 kilovolts, to initiate the discharge. Voltages availablein oil field applications generally do not exceed l kilovolts.

In the present invention, the semiconductor layer provides a shunt pathbetween the electrodes l1, 12. With such a shunt path, applied voltagesof less than l0 kilovolts are required to create an electricaldischarge. It is believed that a sufficient current flow is establishedthrough the shunt path to ionize molecules on the semiconductor surface.When that occurs, a very low resistance electrical path is provided anda spark will be discharged. The invention disclosed herein makes itpracticable to use spark discharge devices in subterranean drillingapplications.

It is believed that a drill constructed in accordance with theseprinciples will function at voltages of l to 3 kilovolts which is withinvoltage range of conventional oil field submergible pumps and electricdrills, and is compatible with conventional electrical cables and otherreadily available equipment.

In those applications in which direct current electrical power must betransmitted down a very long drill string, the associated largecapacitance of the electrical cable may decrease the spark dischargerate. The

operation of the spark drill may be enhanced by'trans-= mission ofalternating current down the drill string and locating circuit means forinitiating thespark discharge-' in the spark drill assembly or as asubassembly which forms a part of the drill string and drillcombination.

Shown in FIG. 3 is a suitable circuit for this-purpose:-

microfarads and V is the charging potential in kilovolts.

Experimental work reports show that energy from 45 to 375 joules isrequired for rock penetration. Thus, a 3 kilovolt power supply to a I00microfarad capacitor would provide sufficient energy.

To this end, FIG.'3 shows an alternating current (a.c.) power supply 20,located at the well surface, couf pled to a direct current power supply21 which is coupled to the electrodes 11, 12 and the shunt p'atli15. Thedirect current power supply 21 is further represented by block 21 inFIG. 1 and constitutes a rectifying circuit and a trigger circuit toinitiate the spark. FIG. 1 further shows a cylindrical container 22 forhousing the electrical circuitry and an electrical connector 23 isprovided as an attach point for an electrical power cable from thealternating current power supply 20.

Turning again to FIG. 3, the 3 kilovolt a.c. power supply 20 is coupledto the trigger circuitry 21 by power cable 24. The trigger circuitrycomprises a rectifying circuit 25, a capacitor 26 and a trigger means 27which may simply be a spark gap having a known breakdown voltage or asolid state control device, either of which may be selected to provideaknown rate of energy pulses to be impressed upon the electrodes l1, 12.

As an alternative embodiment, the spark .drill of FIGS. 1 and 2 is shownin FIGS. 3 and 4 with a plurality of first or center electrodes and alike plurality of direct current power supplies (not all shown). Asshown in FIG. 4, the direct current power supply 21 is coupled to acenter electrode 12 which is spaced from an outer electrode 11 having asemiconductor layer 15 therebetween. Other direct current power supplies21a, 21b are provided to supply current pulses to other electrodes 12a12d shown in FIG. 5. As FIG. 5 indicates, the multiple center electrodesmay be spaced around the lower surface of the spark drill. Thesemiconductor material layer 15 must extend from each of the multipleelectrodes 12, 12a 12d, to the outer electrode 11. The semiconductormaterial need not cover the central area intermediate the centralelectrodes and as shown, the insulating dielectric 14 is exposed. Eachdirect current power supply may be coupled to two or more centerelectrodes or may be provided in a one to one relationship.

WHAT IS CLAIMED IS:

1. A subterranean spark drill comprising:

a. a first electrode;

b. a second electrode;

c. a layer of semiconductor material disposed between said electrodesand providing an electrical shunt path therebetween; and

d. at least one fluid passageway having a port proximate said electrodesand arranged such that a drilling fluidmay be distributed into theregion between said electrodes and the subterranean structure wherebyenergy from a spark discharge 1 between said electrodes will betransmitted through the fluid to the subterranean structure.

2. The spark drill of claim 1 wherein said first electrode substantiallysurrounds said second electrode.

3. The spark drill'of claim 1 wherein said second electrode includes aplurality of radially outwardly extending legs, each said leg beingspaced from said first electrode and each in combination with said firstelectrode providing a spark gap.

4. The spark drill of claim 1 wherein said first electrode issubstantially cylindrically shaped and has an outer diameterapproximately the same, but less than the diameter of the hole to bedrilled and said cylindrical electrode is sufficiently long to include ahousing for associated hardware.

5. The spark drill of claim 1 further comprising a direct currentelectrical power supply coupled to said electrodes.

6. The spark drill of claim 5 wherein said power supply does not exceedan output of kilovolts.

7. The spark drill of claim 1 further comprising circuit means havingthe output coupled to said electrodes for impressing a direct currentvoltage pulse therebetween and for applying the voltage across the shuntpath provided by said layer of semiconductor material, said circuitmeans further having an input which may be coupled to an electricalpower source.

8. The spark drill of claim 7 wherein said circuit means comprises:

a. a capacitor in parallel with said electrodes; and

b. spark initiating means coupled in series between said capacitor andsaid electrodes for initiating said impressing of a voltage pulse onsaid electrodes.

9. The spark drill of claim 7 wherein said power supply has an output ofless than 5 kilovolts, and wherein said capacitor is selected to providedischarge energy in the range of45 400 joules.

10. A subterranean spark drill comprising:

a. a first electrode;

b. a plurality of second electrodes;

c. a layer of semiconductor material disposed between each saidplurality of second electrodes and said first electrode providing shuntpaths therebetween; and

d. at least one fluid passageway having a port proximate said electrodesand arranged such that a drilling fluid may be distributed into theregion between said electrodes and the subterranean structure wherebyenergy from a spark discharge between said electrodes will betransmitted through the fluid to the subterranean structure.

11. The spark drill of claim 10 further comprising:

a. a rectifying circuit having an input that may be coupled to analternating current source; and

b. a trigger circuit having its input coupled to the output of saidrectifying circuit and its output coupled to said plurality of secondelectrodes for impressing a voltage pulse thereupon.

12. The spark drill of claim 1 1 wherein trigger circuit comprises:

a. a capacitor in parallel with said electrodes; and

b. spark initiating means coupled in series between said capacitor andsaid electrodes for initiating and impressing said voltage pulse.

13. The spark drill of claim 10 including:

a. a like plurality of rectifying circuits each having an input whichmay be coupled to an alternating current source; and

b. a like plurality of trigger circuits each having an input coupled tothe output of one of said like pluralityof rectifying circuits, and eachhaving an output coupled to one of said plurality of second electrodesand said first electrode.

14. The spark drill of claim 13 wherein said like plurality of triggercircuits each comprises:

1. A subterranean spark drill comprising: a. a first electrode; b. asecond electrode; c. a layer of semiconductor material disposed betweensaid electrodes and providing an electrical shunt path therebetween; andd. at least one fluid passageway having a port proximate said electrodesand arranged such that a drilling fluid may be distributed into theregion between said electrodes and the subterranean structure wherebyenergy from a spark discharge between said electrodes will betransmitted through the fluid to the subterranean structure.
 2. Thespark drill of claim 1 wherein said first electrode substantiallysurrounds said second electrode.
 3. The spark drill of claim 1 whereinsaid second electrode includes a plurality of radially outwardlyextending legs, each said leg being spaced from said first electrode andeach in combination with said first electrode providing a spark gap. 4.The spark drill of claim 1 wherein said first electrode is substantiallycylindrically shaped and has an outer diameter approximately the same,but less than the diameter of the hole to be drilled and saidcylindrical electrode is sufficiently long to include a housing forassociated hardware.
 5. The spark drill of claim 1 further comprising adirect current electrical power supply coupled to said electrodes. 6.The spark drill of claim 5 wherein said power supply does not exceed anoutput of 10 kilovolts.
 7. The spark drill of claim 1 further comprisingcircuit means having the output coupled to said electrodes forimpressing a direct current voltage pulse therebetween and for applyingthe voltage across the shunt path provided by said layer ofsemiconductor material, said circuit means further having an input whichmay be coupled to an electrical power source.
 8. The spark drill ofclaim 7 wherein said circuit means comprises: a. a capacitor in parallelwith said electrodes; and b. spark initiating means coupled in seriesbetween said capacitor and said electrodes for initiating saidimpressing of a voltage pulse on said electrodes.
 9. The spark drill ofclaim 7 wherein said power supply has an output of less than 5kilovolts, and wherein said capacitor is selected to provide dischargeenergy in the range of 45 - 400 joules.
 10. A subterranean spark drillcomprising: a. a first electrode; b. a plurality of second electrodes;c. a layer of semiconductor material disposed between each saidplurality of second electrodes and said first electrode providing shuntpaths therebetween; and d. at least one fluid passageway having a portproximate said electrodes and arranged such that a drilling fluid may bedistributed into the region between said electrodes and the subterraneAnstructure whereby energy from a spark discharge between said electrodeswill be transmitted through the fluid to the subterranean structure. 11.The spark drill of claim 10 further comprising: a. a rectifying circuithaving an input that may be coupled to an alternating current source;and b. a trigger circuit having its input coupled to the output of saidrectifying circuit and its output coupled to said plurality of secondelectrodes for impressing a voltage pulse thereupon.
 12. The spark drillof claim 11 wherein trigger circuit comprises: a. a capacitor inparallel with said electrodes; and b. spark initiating means coupled inseries between said capacitor and said electrodes for initiating andimpressing said voltage pulse.
 13. The spark drill of claim 10including: a. a like plurality of rectifying circuits each having aninput which may be coupled to an alternating current source; and b. alike plurality of trigger circuits each having an input coupled to theoutput of one of said like plurality of rectifying circuits, and eachhaving an output coupled to one of said plurality of second electrodesand said first electrode.
 14. The spark drill of claim 13 wherein saidlike plurality of trigger circuits each comprises: a. a capacitor inparallel with one of said plurality of electrodes; and b. sparkinitiating means coupled in series between said capacitor and said oneof said plurality of electrodes for impressing a voltage pulsethereupon.